1. Field of the Invention
This invention relates to a system for determining locations by detecting doppler shift in radio signals. If a projectile carries on board a means for continuously transmitting a fixed frequency signal this signal will be received by a ground station as a changing frequency as the relative velocity between the projectile and the ground station changes. This is a classical phenomenon given by the formula: EQU f=fo.+-.(v/c) for
where p0 f=the frequency received by ground station
fo=the fixed frequency of the transmitter PA0 v=the relative velocity between projectile and observer PA0 c=the velocity of light
To utilize this principle for position location, two projectiles carrying transmitters must be fired, at different, known, relative azimuth angles, so their paths are not parallel.
As such a projectile passes an observation post the frequency of the signal received will decrease and pass through the standard frequency transmitted by the projectile and continue to decrease. When the observer receives the same frequency signal as was transmitted it means that the relative velocity between the observer and projectile is zero. This can occur only at one position, that is when the observer and the projectile lie in some reference plane, perpendicular to the horizontal direction of the projectiles flight, the observer further being located on the line of intersection of this reference plane with the surface of the earth. When two projectiles are used there will be two such lines located on the surface of the earth. These two lines will intersect at a point which is the observer's position.
Inherent in this analysis is a means of determining the location of the projectile at any time during its flight so as to fix the location of the reference plane. This is done by timing the projectiles flight and noting the time when the reference frequency is received by an outpost. This information with tables of projectile trajectories will fix the projectile's position along the path of flight.
It has been found that the doppler curves obtained from the parabolic-type trajectory of a projectile have the same fundamental characteristics of doppler curves as a body travelling in a straight line; any given point on the parabola has a corresponding point on the subtrack, X-axis, for which a zero doppler is the same for all observers at the same distance along the subtrack regardless of the observer's distance away from the subtrack. These points of zero doppler correspond to a specific time in the trajectory of the projectile.
In general, it is assumed that the velocity of the projectile is constant which further infers that the direction and velocity do not change. The motion of a projectile is continually changing because of friction of the atmosphere but the speed deviates only slightly from the average. It was found, however, that despite velocity change due to change in direction there was still a unique point of zero doppler for all observers on a specific, unique normal or perpendicular to the subtrack.
Only a single outpost has been considered, however, since each outpost operates independently and does not interfere with any other, a multiplicity of outposts may determine their positions by the firing of only two projectiles.
The system herein described may be used for civil as well as military applications. For example, unmanned sensors may be airdropped at positions not readily accessible by other means. By firing two projectiles and determining the point of zero doppler and the relative time of zero doppler for each sensor position, rough surveying to compute land areas and surveys of natural resources can be quickly and economically determined.
2. Description of the Prior Art
The doppler phenomenon has been used in many systems for many purposes from doppler satellites for navigation to doppler radar for automobile speed traps.
Various systems have been developed using the doppler shift phenomenon of which the following U.S. Patents are representative.
U.S. Pat. No. 3,351,943 by George B. Bush et al appears to be the closest reference of the patented art to the subject invention and is representative of attempts to locate the position of objects on the face of the earth by the use of satellites. This patent teaches the use of a satellite carrying a transmitter wherein the satellite transmits a continuous wave of fixed frequency as it orbits and which beats against a reference signal at the observer station to produce a doppler signal. Ships at sea and other observers who desire the compute their location, monitor the satellite transmissions and when correlation of sequential doppler signals exists the satellite and the observer are closest.
It is recognized that there must be a certain number of satellites in suitable orbits to provide the required information for determining the navigational positions of observers.
The system of this patent has several inherent characteristics which make this system inadequate for military use in locating forward field outposts. First, the satellite has a definite period and its time of availability is available to everyone thus subjecting the system to electronic countermeasures. Second, the satellites are available only during certain times and not at the will of the user. Finally, the satellite is an expensive means for establishing a system intended for localized use and does not lend itself to random reference frequency calibration at the will of the user.
U.S. Pat. No. 3,254,341 by Georges Broussand is similar to the subject invention in that the doppler shift technique is used. This patent teaches use of a satellite carrying a transmitter wherein the satellite transmits a continuous wave of fixed frequency as it orbits. Ships at sea and other observers who desire to compute their location monitor the satellite transmissions and a clock at the observer which is apparently in a phase locked loop with a clock on board the satellite. Successive measurements of the signals doppler shift are made and all the possible positions which could receive such a shift are plotted knowing the satellites path. This technique results in a family of hyperbola which intersect at two points, one of which is the position of the observer.
Since this system operates on such a large expense this ambiguity can easily be resolved by the observer generally knowing in which quadrant of the globe he is. The patent recognizes limitation by stating, "Provided the ambiguity as to the sign of X.sub.o is removed, the point where the signals are received from the satellite can be determined by determining the hyperbola and their points of intersection."
There is another embodiment of this patent wherein a single doppler shift is intercepted and positions plotted. The distance from the satellite to the observer is also plotted. This results in the intersection of a circle and a hyperbola; again two solutions. It is of course recognized that there is one special case when these solutions are exact and that is when the satellite passes directly over the observer.
The system described by this patent is inadequate in that not only does it have all the inherent limitations of all satellite systems as previously mentioned but it also does not give an exact solution. When operating in a localized area an exact solution is necessary. Also, the ambiguity of this system is even more difficult to resolve as these two solutions approach each other, which is the case at points close to earth.
The present system is an improvement over the prior art in that the point of zero doppler and the relative time of the zero doppler for each observer are the only parameters required. These together with the known and controlled geometry of the subtrack, X-axis, of the projectile or missile determine the positions of any number of observers. All of the data required to locate an unlimited number of observers is obtained in only the time required to fire two artillery projectiles or missiles.